The invention relates to an MR arrangement which includes a medical instrument, an RF arrangement which is provided thereon and in which at least one microcoil and a capacitor are connected so as to form a resonant circuit, and also includes an optical signal lead via which a light signal is applied to the RF arrangement.
The invention also relates to a method of localizing a medical instrument in the examination volume of an MR apparatus, which medical instrument includes an RF arrangement in which at least one microcoil and a capacitor are connected so as to form a resonant circuit, and also an optical signal lead via which a light signal is applied to the RF arrangement, the position of the medical instrument being determined from the MR signal generated by the microcoil in the vicinity thereof.
The invention also relates to a medical instrument, notably an intravascular catheter, a guide wire or a biopsy needle, which includes an RF arrangement which is provided at the distal end and in which at least one microcoil and a capacitor are connected so as to form a resonant circuit, and also an optical signal lead via which a modulated light signal is applied to the RF arrangement, and also relates to an MR apparatus provided with such a medical instrument.
The localization of interventional instruments is important in medicine, that is, both in diagnostic and in therapeutic methods. Such instruments may be, for example, intravascular catheters, biopsy needles, minimal-invasive surgical instruments or the like. An important application of interventional radiology is formed by angiography which is intended to uncover the anatomical details of the vascular system of a patient.
Angiography methods based on magnetic resonance tomography (MR) are becoming increasingly more important nowadays. In comparison with X-ray diagnostics as customarily practiced thus far, magnetic resonance offers the major advantage of significantly enhanced tissue selectivity. MR techniques are known in which a microcoil for the detection of magnetic resonance signals is provided on an interventional instrument. Of particular interest in this respect are MR methods for the examination of blood vessels by means of intravascular catheters whose tip is provided with such a microcoil.
An elementary problem encountered in such MR-assisted angiography methods is that long electrical connection leads which extend along the entire length of the intervascular catheter are required so as to transfer the RF MR signal from the microcoil mounted at the tip of the catheter to the electronic receiving circuit of the MR system used. The strong RF radiation in the examination zone may give rise to undesirable and hazardous heating phenomena in such connection wires. The RF fields within the examination zone are capable of generating standing waves in the cables extending within the catheter, thus giving rise to resonant RF heating of the cable. The use of intravascular catheters with long cables extending therein is hampered by second thoughts concerning the safety of such arrangements. The described phenomena can be calculated only with difficulty, because the resonant RF heating is dependent on the field distributions of the RF fields as well as on the geometrical and electrical properties of the electrical leads and their dielectric environment. During experiments sudden intense heating has occurred which could give rise to life-threatening injuries of a patient to be examined.
The risk of resonant RF heating is completely avoided when an optical transmission technique is used for the transmission of the MR signals.
U.S. Pat. No. 6,236,205 B1 discloses an MR device with a medical instrument and a method for localizing the medical instrument in which a resonant circuit which is mounted on the medical instrument is influenced by way of an optical control signal which is applied to the resonant circuit via an optical fiber. At least one microcoil forms part of said resonant circuit. In conformity with the known method first an RF excitation is carried out in a customary manner in the entire examination volume of interest. The nuclear magnetization thus produced in the vicinity of the medical instrument induces a voltage in the microcoil. The resonant circuit thus triggered subsequently emits an RF signal whereby the MR signal in the vicinity of the microcoil is intensified and modified. According to the known method the resonant circuit is alternately switched on and off by way of an optically controllable impedance by variation in time of the optical control signal. As a result, the RF signal emitted by the microcoil also varies in time in conformity with the control signal so that it can be distinguished from the background signal. This feature of distinction is used for determining the position of the medical instrument.
The major drawback of the known method resides in the fact that the magnetization excitation must be excited in the entire examination volume. In order to determine the position, use is made of a difference signal which represents the difference between the MR signals detected while the resonant circuit is switched on and off. In this difference signal the weak local signal of the resonant circuit has to compete with the strong background signal of the entire examination volume. Therefore, the signal-to-noise ratio of the measuring signal available for determining the position is comparatively poor.
It is a further drawback that the known method requires a significant amount of additional measuring time. It is first necessary either to generate complete images or to make repeatedly projections in each of the three spatial directions in order to enable the position of the medical instrument to be found by differentiation. The repetition in the case of the projections is necessary to increase the signal-to-noise ratio of the measuring signal by averaging.
Moreover, it is a drawback that the known method is very susceptible to motional artefacts. When the position of the medical instrument is changed in the course of the measurement, errors are introduced in the image data and reliable localization is strongly impeded.